These layouts are stored in /usr/share/X11/xkb/symbols/chromebook* . To modify the functionality of a key, one can make changes to the appropriate file (as root).
For instance, to remap the power key to Delete, add the following line in the appropriate file: key <POWR> {[ Delete ] };, and set the power button to "Do nothing" in the Power Manager menu.

Key Mappings

Key

Combination

F1-F10

Accessed directly or with Overlay, depending on Keyboard layout

F11

F12

Home

Overlay + Left Arrow

End

Overlay + Right Arrow

Page Up

Overlay + Up Arrow

Page Down

Overlay + Down Arrow

Insert

Overlay + .

Delete

Overlay + Backspace

Caps Lock

Right-Alt as Overlay: Overlay + Left Alt + Right ShiftSuper as Overlay: Overlay + Left Alt